The present invention relates to a rotary hearth furnace (RHF) in which premixed pellets of metal oxides and carbon are transported on a radially moving belt during which time the pellets are exposed to hot combustion gases in a countercurrent manner resulting in the metal oxide being reduced to sponge metal. More particularly, the present invention relates to such a rotary hearth furnace in which oxygen is injected into the furnace such that the carbon monoxide and hydrogen present in the combustion gases are oxidized to generate additional heat for heating the pellets. The injection of the oxygen is accomplished by using an oxy-fuel burner having a flat, wide, flame to increase heat transfer to the pellets. The oxygen is injected at strategic locations within the furnace to combust the exhaust gases and still allow the reduction of the metal oxide.
Doughnut shaped rotary hearth furnaces used to reduce metal oxides are well known in the art. In a traditional rotary hearth furnace heat is provided by air fuel burners placed in the furnace hearth aimed at the top of the pellet bed. The radial belt inside the furnace moves around the circumference of the furnace at a rate sufficient to enable the pellets to reside in the furnace for approximately one complete revolution. Combustion gases are exhausted through a top vent near where the pellets are introduced. Combustion gases in a rotary hearth furnace are often generated from other sources such as the offgas of a smelter.
It is important that the combustion takes place in the most thermally efficient manner in order to conserve both fuel and oxidant as well as in a manner to reduce harmful emissions. It is also important that the furnace atmosphere is sufficient to reduce the metal oxide. However, in conventional rotary hearth furnaces such as described above, there is a loss of efficiency and unwanted emissions due to the partial products of combustion being exhausted out of the furnace. As will be discussed, the present invention provides improvement to rotary hearth furnaces such as those discussed above by utilizing additional oxygen to oxidize certain post-combustion gases which provides additional heat; and to reduce carbon monoxide in the exhaust, while maintaining an appropriate reducing environment at all locations in the furnace so that accelerated oxide reduction can occur.
The present invention pertains to a rotary hearth furnace comprising a cylindrical chamber having substantially parallel cylindrical sidewalls and a top and bottom forming an enclosure to contain a belt and metal oxide/carbon pellets. One or more oxy-fuel burners are positioned to fire inside the enclosure to inject a stream of hot combustion gases to the pellets which flow in a direction counter to the direction of the pellets.
The oxy-fuel burners are preferably flat-flame oxy-fuel burners such as those described in U.S. Pat. Nos. 5,299,929 and 5,360,171 assigned to the BOC Group, Inc. The amount and distribution of oxygen injected into the furnace is dictated by thermodynamic modeling of the furnace with the objective of minimizing the amount of fuel required per ton of metal formed, subject to heat transfer constraints. The burners are optimally placed so that a majority of the oxygen is injected in the first section (or heating zone) of the furnace to create an oxidant rich atmosphere with the remainder of the oxygen injected in the second section (or reducing zone) of the furnace wherein a fuel rich zone is created.
The hot combustion gases should preferably comprise an amount of oxygen in excess of the amount necessary for stoichiometric combustion. In such manner, the energy can be extracted from the carbon monoxide and hydrogen present in the hot combustion gases which would normally be exhausted from the hearth. The use of this method can effect up to a 55% reduction in the amount of natural gas consumed per unit metal output.
A further benefit of the present invention is the decrease in the size of the waste gas handling system due to a decrease in the amount of nitrogen in the waste gas stream.
The present invention can be used in existing rotary hearth furnaces or can be used to design more efficient new furnaces.